Californians can buy cheap rocket parts from Alabama without worrying about exchange rate risk just as Germans can buy cheap olive oil from Greece.

However fiscal transfers between the two states (I.e. taxes -> welfare payments, labor standards etc) mean that the cheaper goods are more likely to be up to standards and that Alabamans can buy California almonds and Roku boxes. (This is one way cutting welfare hurts capitalism). The EU has no such fiscal mechanism thus the process drifts out of a crisis.

Germany’s Länderausgleich is actually more extensive than the US’s, which has made its economy so strong for so long, yet the country is loath to extend its benefits (much of which would flow back to Germany!) to the rest of the EU.